1. Field of the Invention
The present invention relates to an electrometer for measuring surface potential of an object to be measured. The surface potential measuring meter detects the surface potential as an AC signal. The present invention relates also to an electrostatic recording apparatus of the type in which an electrostatic latent image is formed on a recording medium and then the latent image is visualized. The recording apparatus is provided with the surface potential measuring meter to measure the surface potential of the formed latent image. Also, the present invention relates to such electrostatic recording apparatus in which the surface potential of latent image is measured and various conditions for image formation are controlled by means of the measured output.
2. Description of the Prior Art
To detect the surface potential of an object to be measured as AC signal, there has been used hitherto a chopper which intermittently blocks the path between the object and a measuring electrode. When a vibrator such as a tuning fork is used as the chopper, the resonance frequency is determined depending upon the length and width of the vibrator then used. To keep the resonance frequency constant for all the vibrators thus used, an extremely high preciseness in machining the vibrators is required. This requirement is practically difficult to satisfy. As a result, the resonance frequency and therefore the phase of the detected AC signal varies undesirably from one vibrator to another. If a band-pass filter is used to filter off noise from the detected AC signal, then an undesirable variation in phase of the AC signal will be caused also by the irregularity in the quality of parts of which the filter is constituted.
DC type surface potential measuring meters are also known in the art. For example, U.S. Pat. Nos. 4,100,484; 4,106,869 and 4,063,154 have disclosed feedback type surface potential measuring meters. In this type of known surface potential measuring meter, the detected voltage induced in a measuring electrode is amplified by a high voltage amplifier circuit and the same voltage as the measured one is fed back to the housing of the measuring electrode. This type of electrometer has an advantage that the potential difference between the meter and the object remains always at 0 V and therefore it is made possible to detect the surface potential without being affected by the distance between the meter and the object to be measured.
However, the known meters also have some disadvantages. Generally speaking, the feedback control system is apt to become unstable and oscillate, in particular when the open-gain by the high voltage amplifier is very large. For example, when a DC-DC inverter is used as the amplifier, the inverting transformer suddenly becomes unstable in oscillation and tends to stop when the input voltage is dropped to a certain level. This results in an abrupt drop in output because the linearity of the input/output characteristic is lost. This is equivalent to an abrupt increase of gain by amplification. Consequently, if negative feedback is applied at this point, then the negative feedback system becomes unstable causing oscillation of the detection output.
In prior art, various electrostatic recording apparatuses have been proposed in which the potential of latent image formed on a recording medium is detected and the detected signal is used to control the conditions necessary for image formation. For example, reference is made of U.S. Pat. Nos. 3,788,739 and 3,438,705.
For this known type of electrostatic recording apparatus it is known that stability and reliability of control on the image formation can be improved very much by treating the control data employing a digital computer. However, use of a microcomputer for sequence control involves some difficulties. When the function of controlling potential is added to the microcomputer, the number of input-output ports allocable for the potential control is very small since the greater part of the input-output ports must be allocated for sequence control of key entry, display and various image forming means. Therefore, the control of potential, if made possible, is limited within a very narrow range and only uniformalized controls can be realized employing such microcomputer.
It is also expected that stability and reliability of control on the above mentioned type of electrostatic recording apparatus can be further improved by controlling the output of the high voltage charger employing a digital computer. However, the output of such digital computer is unsteady and variable depending upon the initial state until the computer has been initialized, that is, until its output port has been reset. In this unstable state of the output it is required to keep the high voltage charger off. Otherwise, an unsteady control voltage may be applied to the high voltage charger and thereby the recording medium may be brought into an undesired condition.
Use of a digital computer in the arithmetic operation of the measured surface potential will also bring forth substantial improvement in stability and reliability of control on the electrostatic recording apparatus. However, in this case, if any trouble occurred in the electrometer or in the potential measuring circuit or in its A/D converter, then the digital computer would no longer receive the correct measured data of surface potential. As a result, the output for controlling the high voltage charger or for controlling the bias voltage for development would be an abnormal value which may produce images of poor quality.
If the high voltage output from the high voltage charger becomes abnormally high, then the corona discharge is changed into a glow discharge which may damage the surface of the recording medium and/or the high voltage generator of the charger itself. For this reason it is a common practice in the art to provide a breaker in the apparatus. When the input to the high voltage generator exceeds a certain predetermined value, the breaker inhibits the generator from issuing any output to prevent the damage. However, actuation of such breaker causes interruption of the image forming operation within the apparatus. If such breaker is frequently actuated, the reliability of the apparatus may be lost completely.